Metal complexes of heterocyclic aromatic compounds

ABSTRACT

A novel metal complex of a heterocyclic aromatic compound which shows a low activation energy, is stabilized structurally, is capable of modulation of the structure thereof, and capable of preferably functioning as a molecular device in technological fields. The metal complex of a heterocyclic aromatic compound comprises a transition metal (for example, silver ion) as a central atom, and basic ligands comprised of a 5-membered heterocyclic aromatic compound (for example, pyrrole rings), in which the position of the central atom can be changed by an internal factor such as transfer of an electric charge or by an external factor such as application of an electric field, a change in acidity of the surrounding environment, etc., whereby the number of atoms (or the number of electrons) relating to the coordination ability can be modulated, and, upon polymerization, the conformation can be modulated depending on the position of the central atom.

TECHNICAL FIELD

[0001] The present invention relates to a metal complex of aheterocyclic aromatic compound.

BACKGROUND ART

[0002] In recent years, an increasing interest has been taken inmolecular devices learnt from excellent structures and functions seen inorganisms. Most of vital reactions pertain to physical and chemicalprocesses through which proteins, such as enzymes, or functionalcoloring matter molecules pass at high efficiency and high selectivity.A lot of researches have been made in order to develop a new moleculardevice by artificially modeling after such a function.

[0003] However, as compared with the today's progress of the researchesof organisms themselves such as genome decoding and elucidation of thefunctions of proteins, few novel molecular devices have appeared whichare learnt from the functional principles in organisms. In practice,most of the researches are directed not toward the realization of amolecular device having a function learnt from an organism but towardthe studies of a fabrication (orientation, arrangement, laminate film,or the like) which relates to a new device having the function not yetproduced but with which the device can possibly be produced.

[0004] Accordingly, it is an object of the present invention to providea novel metal complex of a heterocyclic aromatic compound which makes itpossible to learn from the functional principles of enzymes in organismsand, particularly, to achieve favorable functions as molecular devicesin technological fields.

DISCLOSURE OF INVENTION

[0005] The present invention pertains to a metal complex of aheterocyclic aromatic compound, comprising an acidic central atomcomprised of a metal (for example, an acidic ion of a transition metal,such as Ag, as a central metal), and basic ligands comprised of a5-membered heterocyclic aromatic compound, wherein the position of thecentral atom can be varied (particularly, the number of atoms (or thenumber of electrons) relating to the coordination ability can beregulated within the range of 1 to 5) by an internal factor such astransfer of an electric charge or an external factor such as applicationof an electric field.

[0006] According to the metal complex of a heterocyclic aromaticcompound according to the present invention, the position of the centralatom (namely, the number of atoms (or the number of electrons) relatingto the coordination ability) can be regulated, and an original unstablestructure can be automatically stabilized. This resembles the functionalprinciple of an enzyme in an organism, causes a modulation of thestructure of a derivative (inclusive of polymer) of the metal complexaccording to the change in the position of the central atom, therebystabilizing the structure, and, particularly, can realize favorablefunctions as molecular devices in technological fields.

BRIEF DESCRIPTION OF DRAWINGS

[0007]FIG. 1 is a schematic diagram of a pyrrole molecule in anembodiment of the present invention.

[0008]FIG. 2 is a schematic diagram of a pyrrolidine molecule in anembodiment of the present invention.

[0009]FIG. 3 is a schematic diagram of a pyrrole-pyrrole molecule in atrans-isomer having nitrogen atoms in symmetric positions in anembodiment of the present invention.

[0010]FIG. 4 is a schematic diagram of a pyrrole-pyrrole molecule in acis-form having nitrogen atoms on the opposite sides in an embodiment ofthe present invention.

[0011]FIG. 5 is a schematic diagram of a trans-isomer of a pyrroleoligomer in an embodiment of the present invention.

[0012]FIG. 6 is a schematic diagram of a cis (gauche)-isomer of apyrrole oligomer in an embodiment of the present invention.

[0013]FIG. 7 is a schematic diagram of a cis-trans mixture of a pyrroleoligomer in an embodiment of the present invention.

[0014]FIG. 8 is a schematic diagram of an example in which the structureis stabilized by a regulating factor notwithstanding the dihedral angleis near 135°, in an embodiment of the present invention.

[0015]FIG. 9 is an absorption spectrum diagram of a complexed pyrrolemolecule in an embodiment of the present invention.

[0016]FIG. 10 is a graph showing the concentration of silver ionnecessary for forming a complex, in an embodiment of the presentinvention.

[0017]FIG. 11 is a graph showing the concentration ratio of silver ionand pyrrole monomer necessary for oxidative polymerization, in anembodiment of the present invention.

[0018]FIG. 12 is a UV-visible absorption spectrum diagram of a solutionof trimeric pyrrole cations, in an embodiment of the present invention.

[0019]FIG. 13 is a UV-visible absorption spectrum of a pyrrole oligomersolution containing only the trimer, in an embodiment of the presentinvention.

[0020]FIG. 14 is a UV-visible absorption spectrum diagram of a pyrroleoligomer solution containing only the nonamer, in an embodiment of thepresent invention.

[0021]FIG. 15 is an absorption spectrum diagram of an N—H structureportion (str.) of a secondary amine salt, in an embodiment of thepresent invention.

[0022]FIG. 16 is a graph showing variations in wave number andabsorption intensity due to charge amount at a C—N structure portion(str.) on the side on which —OH group of a secondary amine salt ispresent, in an embodiment of the present invention.

[0023]FIG. 17 is an absorption spectrum diagram of a C—N structureportion (str.) on the side on which —OH group of a secondary amine saltis absent, in an embodiment of the present invention.

[0024]FIG. 18 is a graph showing variations in absorption intensity ofan N—H structure portion (str.) of the pyrrole ring which has astructural strain and in which the nitrogen atom is protonated, in anembodiment of the present invention.

[0025]FIG. 19 is a schematic diagram showing the case where silver ispresent on a 1-site carbon atom on the side on which —OH group ispresent, in an embodiment of the present invention.

[0026]FIG. 20 is a schematic diagram showing the case where silver ispresent on an α-site carbon atom on the side on which —OH group ispresent, in an embodiment of the present invention.

[0027]FIG. 21 is a schematic diagram showing the case where silver ispresent on a nitrogen atom, in an embodiment of the present invention.

[0028]FIG. 22 is a schematic diagram showing an example in which silveris disengaged from a pyrrole ring, in an embodiment of the presentinvention.

[0029]FIG. 23 is a schematic diagram of another example in which silveris disengaged from the pyrrole ring, in an embodiment of the presentinvention.

[0030]FIG. 24 is a graph showing the relationship between the variationin transmittance for white light and memory time, of a metal complex ofpyrrole based on the present invention, in an embodiment of the presentinvention.

[0031]FIG. 25 is a graph showing the variation in absorption spectrumwhen the charge state of a metal complex is varied, in an embodiment ofthe present invention.

[0032]FIG. 26 is a graph showing the variation in absorption spectrumwhen the charge state of a metal complex is varied, in an embodiment ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0033] Now, the present invention will be described more specificallybased on embodiments thereof.

[0034] In the metal complex of a heterocyclic aromatic compound based onthe present inventin, the 5-membered heterocyclic aromatic compounddesirably contains an N (nitrogen) atom. In addition, the N atom isdesirably protonated or having an sp3 hybrid orbital. Furthermore, asubstituent group capable of intramolecular hydrogen bonding may bepresent on a β-site carbon atom of the heterocyclic aromatic compound.

[0035] Here, examples of the substituent group capable of intramolecularhydrogen bonding include —OH group, etc. Generally, a method ofprotonation for cutting away a double bond of carbon is easier to carryout, but the method is unfavorable because it hinders polymerization ora π-electron conjugated system. Therefore, in the present invention, adesirable method for putting the above-mentioned atom into the sp3hybrid orbital is a method in which a substituent group capable ofintramolecular hydrogen bonding, for example, an —OH group is introducedonto the β-site carbon atom of the heterocyclic aromatic compound,whereby predominance is given to coordination of metal ion to thenitrogen atom.

[0036] In addition, the transition metal ion is desirably acidic.Examples of the transition metal ion include the ions of atoms of Groups1B to 5B of the periodic table; particularly, Ag is preferable, and Cuand Au may also be used. Furthermore, atoms such that the number ofelectrons in the d-orbital becomes 10 upon ionization may also be used,and examples of such atoms include Zn, Cd, Hg, etc.

[0037] The internal or external factor is indispensable for changing theposition of the above-mentioned central atom so as thereby to obtain theabove-mentioned effect. The internal factor is preferably transfer of anelectric charge, whereas the external factor is preferably applicationof an electric field, a change in the acidicity of the surroundingenvironment, or the like. In this case, for example, the transfer of anelectric charge is preferably |0 to 60| mC/cm² by a redox reaction, theapplication of an electric field is preferably application of a DC or ACvoltage of |0.001 to 100| V between both electrode plates, and theacidicity is preferably pH=0.01 to 14.

[0038] Pyrrole-Metal Complex

[0039] <Basic Structure of Metal Complex>

[0040] The metal complex of a heterocyclic aromatic compound accordingto the present invention is, specifically, desirably a metal complex ofpyrrole or a metal complex of a derivative thereof.

[0041] Here, the derivative is a concept including compounds in which afunctional group is introduced to the heterocyclic ring and alsocompounds in which pyrrole itself or other molecule, atomic group or thelike is substituted for or added to a part of pyrrole.

[0042] <Pyrrole and Derivatives Thereof>

[0043] Next, the metal complex of a heterocyclic aromatic compound basedon the present invention will be described more in detail.

[0044] The following structural formula (1) and FIG. 1 show a pyrrolemolecule, which is a 5-membered heterocyclic aromatic compoundcontaining one nitrogen atom.

[0045] The pyrrole molecule has a structure in which the lone pair ofthe nitrogen atom is non-localized on the aromatic ring, so that itshows little basicity (pKb: 0 to 2.5×10⁻¹⁴). Incidentally, thederivatives of pyrrole in many cases constitute a basic matter of anorganism, as in the cases of hemoglobin, chlorophyll and alkaloid. Inaddition, as is clear from the structural formula (1) and FIG. 1,pyrrole has aπ-electron conjugated system containing two electrons ofthe nitrogen atom, and is easily polymerized to be an oligopyrrole orpolypyrrole.

[0046] The following structural formula (2) and FIG. 2 show apyrrolidine molecule.

[0047] The pyrrolidine molecule shows a strong basicity (pK_(b)˜10⁻³),and can be produced by reducing the pyrrole molecule. The pyrrolidinemolecule shows the properties of a chain secondary amine becauseelectrons of the nitrogen atom take an sp3 hybrid orbital, and can forma salt which will easily be crystallized, but it cannot be polymerizedbecause it has no double bond.

[0048] As above-mentioned, the pyrrole molecule intrinsically has such aproperty that the π-electron conjugated system is varied variously uponsome changes in structure as shown in the structural formulas (1) and(2) and FIGS. 1 and 2, and can easily be polymerized. Besides, it isknown that in an oxidized state of a polymeric film of the pyrrolemolecule, a band is formed due to the polaron movement of the conjugatedsystem, resulting in the presence of conductivity. Hitherto, however,attention has been paid only to the properties of the pyrrole polymer asthe conductive polymer, and a research of the pyrrole polymer as aremarkable functional device has not been conducted (Even in dealingwith the pyrrole polymer as an electrochromic device, only the highdegree of conductivity is utilized.).

[0049] Other 5-membered heterocyclic aromatic compounds than pyrroleinclude thiophene which contains a sulfur atom in place of the nitrogenatom and furan which contains an oxygen atom in place of the nitrogenatom. These have an electron affinity energy and an ionization potentialwhich are substantially equal to each other, so that they can developconductivity due to both cation doping and anion doping. Therefore,thiophene and furan can be used as each of both electrodes, and theyhave been studied principally in relation to battery. However, thepyrrole molecule has an electron affinity energy of 0.3 eV, which isextremely small as compared with its ionization potential of 3.8 eV, sothat it cannot be subjected to cation doping. Therefore, the pyrrolemolecule has not been regarded as much important, even in the researchfields relating to battery.

[0050] Pyrrole Oligomer

[0051] A pyrrole oligomer formed by polymerization of the pyrrolemolecule tends to take the π-bond in the molecule plane in order toenhance the conjugated system. Therefore, for example, a pyrrole-pyrrolemolecule (dimmer) as a pyrrole oligomer is stable when it takes the formof a trans-isomer having the nitrogen atoms at symmetric positions asshown in FIG. 3 and when it takes the form of a cis-isomer having thenitrogen atoms on the opposite sides as shown in FIG. 4. Here, incomparison of the trans-isomer and the cis-isomer with each other, ithas been reported that the trans-isomer has a lower energy and isstabler in the case where one molecule is present in vacuum, but thecis-isomer is formed more easily depending on the surrounding solventmolecules, electrolyte or the like (“First evidence of crystallinestructure in conducting polythiophene”, F. Garnier, et al., J. Mater.Science, 20 (1985) 2687-2694.).

[0052] Here, where the dihedral angle in the trans-isomer is 0° and thedihedral angle in the cis-isomer is 180°, a structure with a dihedralangle of 90° is high on an energy basis and is therefore instable.Namely, when it is intended to achieve a conformation change from thetrans-isomer to the cis-isomer, it is necessary to go over the highenergy mount of the instable structure.

[0053] As to the dihedral angle, the same properties are shown even whenan oligomer with an increased number of pyrrole molecules is formed. Itshould be noted here that, as shown in FIGS. 5 to 7, in the cases of forexample the octamer and higher oligomers, the cis-isomer with a dihedralangle of 180° suffers a steric hindrance, so that a structure with adihedral angle of about 140 to 160° is stable, and a spiral structure isformed; besides, a mixture of trans-isomer and cis-isomer may also beformed in some cases.

[0054] The present inventors, paying attention to substances such as thepyrrole molecule which have basicity and the specificity of a conjugatedsystem, have made intensive and extensive studies for synthesizing afunctional molecule based on a novel principle (for example, those withthe dihedral angle varied variously), and, as a result of their studies,have found out the metal complex of a heterocyclic aromatic compoundbased on the present invention.

[0055] The metal complex of a heterocyclic aromatic compound based onthe present invention is, for example, the above-mentioned pyrrole-metalcomplex (in which 0<n≦5, i.e., the number n of atoms relating to thecoordination ability is an integer of 1 to 5, which can be defined asthat the number of electrons relating to the coordination ability is inthe range of 1 to 6) or the like in which the above-mentioned basicligands comprised of the 5-membered heterocyclic aromatic compound arein coordination to the silver ion as the transition metal ion and inwhich, preferably, an —OH group as the substituent group capable ofintramolecular hydrogen bonding is bonded to the 1-site carbon atom ofthe heterocyclic aromatic compound.

[0056] Mechanism of Pyrrole Complexing

[0057] <Complexing Reaction>

[0058] In the complexing, for example, the pyrrole molecule representedby the above structural formula (1) is used as the starting material,the nitrogen atom is protonated by controlling the proton concentrationto change the basicity, thereby forming a pyrrole derivative, and thepyrrole derivatives are put into coordination to the transition metalion, for example, the silver ion to form the complex. For example, 1 molof silver ions can be bonded to 2.5 mol of pyrrole molecules. It shouldbe nodded here that, depending on the degree of protonation, thenitrogen atom is in the sp3 state and the basicity is so strong that thesilver ion is put into coordination bond to that portion, resulting inthe formation of a complex in which the sp3 state is stabilized.

[0059] In this case, since the complex with strong basicity forms acomplex with strong polarization, the position change of the silver ion(namely, the regulation of the number of atoms (or the number ofelectrons) relating to the coordination ability) due to theabove-mentioned internal or external factor cannot be achieved, and,since the double bond is lost, polymerization cannot be achieved.Therefore, in the protonation, complete protonation is not conducted.That is, it is desirable that the reaction is not brought to such astage as to form the pyrrolidine molecule represented by the abovestructural formula (2). Incidentally, it is generally easier to effectthe protonation by breaking up the double bond, but it is unfavorablebecause it produces a bad effect on the polymerization or the π-electronconjugated system. Therefore, as the method for protonation of thenitrogen atom, the above-mentioned substituent group capable ofintramolecular hydrogen bonding, for example, the —OH group issubstituted onto the β-site carbon atom of the heterocyclic aromaticcompound, whereby the sp3 property of the nitrogen atom is enhanced.

[0060] Namely, it is a characteristic feature that the transition metalion, for example, the silver ion is put into coordination to the pyrrolederivatives, and the position of the silver ion is changed in themolecule by the above-mentioned internal or external factor, and thenthe steric structure can be changed.

[0061] <Effect of Internal or External Factor>

[0062] Attention should be paid to the fact that the metal complex of aheterocyclic aromatic compound based on the present invention has such aproperty that the number of atoms (or the number of electrons) relatingto the coordination ability can be regulated by the above-mentionedinternal or external factor.

[0063] Namely, for example, the conformation can be changed byexternally modulating the electron state of the pyrrole oligomer.Specifically, although the possible values of the dihedral angle arestable only in the two forms of the trans-isomer and the cis-isomer(thermal fluctuations of about ±10° are present, though), a high energystate such as that with a dihedral angle of about 90° can also bestabilized by the interaction between the acidic transition metal ion(Ag⁺), which is the modulation factor, and the pyrrole oligomer.

[0064]FIG. 8 is a schematic diagram showing one example in which astructure is stabilized by the modulation factor (Ag⁺) even though thestructure has a dihedral angle of about 135°.

[0065] Method of Producing Pyrrole Oligomer

[0066] <Production of Pyrrole-Metal Complex>

[0067] Next, one example of the method of producing a pyrrole-metalcomplex as the metal complex of a heterocyclic aromatic compound basedon the present invention will be described.

[0068] Pyrrole monomer is mixed with a propylene carbonate solution sothat its concentration is 0.1 mol/l. For the purpose of substitution ofa hydroxyl group onto the β-site of the pyrrole molecule, 1% by weightof water is added to the mixed solution. Since the dissolved oxygenplays an important role, the solution is not subjected to bubbling withnitrogen gas, bubbling with argon gas or the like.

[0069] Then, for protonation of the nitrogen atom in the pyrrolemolecule, an acid is gradually added to the solution so as to graduallyraise the proton concentration. The acid may be HClO₄ or HCF₃SO₃, whichis dissociated as a strong acid in the propylene carbonate solution.Here, the protonation of the nitrogen atom owing to the protonconcentration can be easily monitored through infrared absorptionspectrum. For example, in order to provide such a basicity as to form acomplex, it suffices to pay attention to the spectrum near 2300 cm-₁,which is the N—H stretching vibration band of the secondary amine salt.

[0070] <Production of Pyrrole Oligomer>

[0071] Subsequently, polymerization of the pyrrole ring for complexingwith the transition metal and a pyrrole ring lower in basicity than thejust-mentioned pyrrole ring and an ordinary pyrrole ring is conducted.

[0072] The ionization potential of the pyrrole molecule having basicityis inversely proportional to the basicity; as the basicity is higher,the ionization potential is lower and the polymerization energy islower, so that the polymerization takes place more easily. When thepolymerization is conducted taking this into account, the pyrroleoligomer having a desired structure can be produced.

[0073] For example, silver ion is used as an oxidizing agent for use inpolymerization (oligomerization) of pyrrole and as the above-mentionedtransition metal ion. When the basicity of the pyrrole ring ispreliminarily adapted to the acidity of the silver ion, a complex can beeasily formed. (For example, the silver ion is preliminarily subjectedto aqueous solvation, and the water ligands are subjected to ligandexchange with pyrrole rings. Since water is a hard base, it is easilyreplaced by a soft base when brought into contact with the soft base.)The silver ion suitable for the oxidative polymerization and the complexformation can be determined by the concentration ratios among the silverion, the water molecule and the pyrrole monomer.

[0074] <Confirmation of Pyrrole Oligomer>

[0075] The formation of the pyrrole complex with silver can be confirmedby a steep peak shape near 2300 cm⁻¹ of the amine salt and theabsorption intensity thereof, as shown in FIG. 9. In FIG. 9, the peakintensity varies with the Ag concentration, as indicated by symbols a tod.

[0076]FIG. 10 is a graph showing one example of silver ion concentrationnecessary for the complex formation. It is seen that when the pyrroleconcentration is 0.1 mol/l and AgPF₆ is used, it is preferable to setthe concentration of AgPF₆ to be not less than 0.02 mol/l.

[0077]FIG. 11 is a graph showing the concentration ratio between silverion and pyrrole monomer necessary for the oxidative polymerization. Itis seen that it is preferable to set the concentration ratio to be notmore than 10.

[0078] In addition, the number of the pyrrole rings polymerized can bechecked through visible absorption spectrum, and the number can beeasily confirmed by the difference in π-π* transition energy band gap.The number of the pyrrole rings to be polymerized can be controlled byappropriately varying the kind and concentration of the electrolyte,reaction time, and, further, the density and viscosity of the solvent.Naturally, the number may be controlled chemically. (Reference: S.Martina, V. Enkelmann, A-D. Schluter and G. Wegner, Synth. Met., 51, 299(1992))

[0079]FIG. 12 is a graph showing a UV-visible absorption spectrum of amixed solution of a pyrrole oligomer in the form of a trimeric cation(AgPF₆: 0.04 mol/l; pyrrole: 0.10 mol/l; reaction time: 72 hr).

[0080] In addition, FIG. 13 is a graph showing a UV-visible absorptionspectrum of a pyrrole oligomer solution containing only the trimer(AgBF₄: 0.10 mol/l; pyrrole: 0.1 mol/l; reaction time: 192 min).

[0081] Further, FIG. 14 is a graph showing a UV-visible absorptionspectrum of a pyrrole oligomer solution containing only the nonamericcation (AgNO₃: 0.04 mol/l; pyrrole: 0.1 mol/l; reaction time: 100 hr).Conformation Modulation of Pyrrole Oligomer

[0082] <Influence of Internal or External Factor>

[0083] Next, the activities of silver ion when conformation modulationdue to the above-mentioned internal or external factor is caused whileusing the metal complex of a heterocyclic aromatic compound based on thepresent invention, for example, a pyrrole oligomer were observed throughinfrared absorption spectrum. FIGS. 15 to 18 show the variations inabsorption intensity when, for convenience, electric charges aregradually added starting from an oxidized state, although the reactionis reversible.

[0084]FIG. 15 is a graph showing the variations in absorption intensityof an N—H structure portion (str.) of a secondary amine salt complexedwith silver. As is clear from FIG. 15, as the amount of electrons addedis increased, the N—H structure portion of the complexed pyrrole ringundergoes steric hindrance, and flexibility is gradually lost. However,relaxation does not take place.

[0085] In addition, FIG. 16 is a graph showing the variations inabsorption intensity of a C—N structure portion of a secondary aminesalt complexed with silver on the side on which the —OH group ispresent.

[0086] Further, FIG. 17 is a graph showing the variations in absorptionintensity of a C—N structure portion of a secondary amine salt complexedwith silver on the side on which the —OH group is absent. As is clearfrom FIG. 17, as the amount of electrons added is increased, a sterichindrance is caused, although there is no bond to the C atoms on theside on which the —OH group is absent.

[0087] Besides, FIG. 18 is a graph showing the variations in absorptionintensity of an N—H structure portion of a pyrrole ring having astructural strain.

[0088] As is clear from FIGS. 15 to 18, an increase in absorptionintensity indicates an increase in the concentration of the matter inconsideration itself or an increase in asymmetry. The shift of avibration absorption band in an infrared absorption spectrum to the sideof higher wave number (the shift to the side of higher energy) indicatesthat the intrinsic vibration constant itself is increased (improvementof conjugated system and strain of structure), irrespective of hindranceor relaxation, and/or that the weights of both ends of a functionalgroup are reduced.

[0089] <Discussion of Structural Change and Evaluation of PhysicalProperties>

[0090] Based on the results obtained from the movements of the molecularstate generating the absorption intensity variation and the wave numbershift as above, a computation for structural optimization was conducted,the results being shown below. It is remarked that the experimentalresults are a fact, but they are computational results and rather one ofinterpretations, so that the results are not yet fully established.

[0091] The silver ion is initially present on the β-site carbon atom onthe side where the —OH group is present, as shown in FIG. 19. Thenitrogen atoms of the pyrrole molecules disposed on both sides of thecomplexed pyrrole molecule as the metal complex of a heterocyclicaromatic compound based on the present invention move to the positionsnearest to the silver ion in order to bond to the silver ion. In thisinstance, the dihedral angle formed by the complexed pyrrole moleculeand the pyrrole molecules on both sides thereof is close to that in thestate of the trans-isomer.

[0092] When electric charges are added to the state of FIG. 19, thesilver ion moves onto the α-site carbon atom on the side on which the—OH group is present, and the dihedral angle formed with the pyrrolemolecules on both sides takes such a value that the nitrogen atom isdisposed nearest to silver, as shown in FIG. 20. Although the dihedralangle formed by the complexed pyrrole molecule and the pyrrole moleculeson both sides thereof is in the state of the trans-isomer, the side onwhich the —OH group is absent is not affected by the unpaired electronof oxygen, so that the trimer is stabilized at an angle of 90°.

[0093] When an electric charge is further added, the silver ion movesonto the nitrogen atom as shown in FIG. 21, and, thereafter, it slipsout to the side opposite to the side on which the —OH group is present.The dihedral angle formed by the complexed pyrrole molecule and thepyrrole molecules on both sides thereof is close to that in the state ofthe cis-isomer.

[0094] The dihedral angle formed by the complexed pyrrole molecule andthe pyrrole molecules on both sides thereof always takes such a valuethat the nitrogen atom is disposed nearest to the silver ion. Then, thedihedral angle formed by the complexed pyrrole molecule and the pyrrolemolecules on both sides thereof is now free of influence of the oxygenatom on both sides, so that the trimer is twisted symmetrically and isstabilized at an angle of about 45°.

[0095] In addition, as shown in FIGS. 22 and 23, when the silver ion iscompletely set aside from the complexed pyrrole molecule, all of thethree pyrrole molecules bonded to each other tend to take a conjugatedsystem in a plane at equal distance from the silver ion, and isstabilized in that state.

[0096] From the foregoing, it has been shown that the conformation maybe modulated from the trans-isomer to the cis-isomer by the control ofthe electric charge, and the bonding conditions may be varied so thatthe instability of the structure is stabilized by the silver ion.Specifically, the number n of atoms relating to the coordination abilityof pyrrole can be varied in the range of 0<n≦5.

[0097] Accordingly, the metal complex of a heterocyclic aromaticcompound based on the present invention is characterized in that it hasa multiplicity of kinds of stable positions, instead of one kind ofstable position.

[0098] In addition, the metal complex of a heterocyclic aromaticcompound based on the present invention can show variations in color ofvisible absorption spectrum even after formation of a film, since theconjugated system therein can be modulated.

[0099]FIG. 24 is a graph showing the relationship between variation intransmittance for white light of a film of the metal complex of aheterocyclic aromatic compound and memory time. It is seen from thefigure that various transmittances can be obtained depending on theconformation of the pyrrole rings, and that a memory function ispresent.

[0100] Besides, FIGS. 25 and 26 are graphs showing the variations inabsorption spectrum when the charge state is varied. Incidentally, theaxis of abscissas corresponds to the energy of UV and visible rays; forexample, 2 eV means light with a wavelength of 620 nm, and 4 eV meanslight with a wavelength of 310 nm.

[0101] Here, the control of electric charge amount (the transfer ofelectric charge) may be mentioned as the internal factor, and theequivalent phenomenon can be observed upon application of an electricfield as the external factor. In addition, the position of the silverion or the conformation can be modulated also by variation of pH(variation of acidity of the surrounding environment).

[0102] According to the metal complex of a heterocyclic aromaticcompound based on the present invention, the position of the silver ioncan be modulated by the internal factor, such as transfer of electriccharge, or the external factor, and the intrinsically unstable structurecan be stabilized by the change of the position of the silver ion.Therefore, the metal complex of a heterocyclic aromatic compound basedon the present invention can preferably function as a molecular devicein technological fields.

[0103] In addition, the technological fields to which the metal complexis applied are not particularly limited. For example, use for a displayutilizing variations in the color of emitted light may be mentioned asone example of the technological field, since the number of atoms (orthe number of electrons) relating to the coordination ability can beregulated by the internal or external factor, and other examples of thetechnological fields include a molecular memory material utilizing theability to change the structure, a refractive index modulation device,an analog switch, molecule recognition, an ion permeable film, etc.

[0104] Now, the present invention will be described more in detail byway of examples, but the invention is not limited to the examples.

EXAMPLE 1

[0105] An amorphous ITO film (sputtered at low temperature and enhancedin planarness, having a resistance of 200 Ω/□) was formed on a glasssubstrate measuring 10 mm by 30 mm with a thickness of 1.1 mm. Leadportions are formed at ends of the film, and the assembly was placed ina glass tank.

[0106] A mixed solution prepared by dissolving 0.04 mol/l of silverhexafluorophosphate and 0.10 mol/l of pyrrole in a propylene carbonatesolution (containing 1% by weight of water) was obtained as anelectrolytic solution in the glass tank. The solution was mixed well,and was left to stand for 72 hr.

[0107] A platinum substrate was disposed as a counter electrode in thesolution, and an electric current of 2 mA was galvanostatically passedthrough the entire body of the solution from a driving circuit until theamount of electricity passed reached 50 mC/cm². As a result, anelectrolytically polymerized film of a pyrrole-silver complex in anoxidized state (the metal complex of a heterocyclic aromatic compoundbased on the present invention) was formed on the ITO film. Thepolymerized film did not have a maximum absorption in the visible range,and was colorless (pale gray).

[0108] Next, the substrate was placed in a glass tank containing anelectrolytic solution obtained by dissolving 1 mol/l of tetraammoniumtetrafluoroborate in a propylene carbonate solution bubbled withnitrogen, and an electric current of −1 mA was galvanostatically passedthrough the solution until the amount of electricity passed reached 4mC/cm², upon which the polymerized film turned light pink.

[0109] Subsequently, an electric current of −1 mA was furthergalvanostatically passed until the amount of electricity passed reached4 mC/cm², upon which the polymerized film turned light green.

[0110] Next, an electric current of −1 mA was further galvanostaticallypassed until the amount of electricity passed reached 4 mC/cm², uponwhich the polymerized film turned light orange.

[0111] Subsequently, an electric current of −1 mA was furthergalvanostatically passed until the amount of electricity passed reached4 mC/cm², upon which the polymerized film turned yellow.

[0112] In addition, when the polymerized film was left to stand in eachcolored state in a nitrogen atmosphere, the polymerized film retainedthe initial colored state even after several days.

COMPARATIVE EXAMPLE 1

[0113] A polymerized film was produced in the same manner as in Example1, except that tetraammonium tetrafluoroborate was used in place ofsilver tetrafluorophosphate as an electrolyte in the glass tank. Thepolymerized film was deep reddish brown in color.

[0114] Next, the substrate was placed in a glass tank containing anelectrolytic solution obtained by dissolving 1 mol/l of tetraammoniumtetrafluoroborate in a propylene carbonate solution bubbled withnitrogen, and an electric current of −1 mA was galvanostatically passedthrough the electrolytic solution until the amount of electricity passedreached 4 mC/cm². Upon the passage of the current, the polymerized filmmomentarily turned light pink, but it immediately returned to theoriginal deep reddish brown.

[0115] Subsequently, an electric current of −1 mA was furthergalvanostatically passed until the amount of electricity passed reached4 mC/cm², upon which the polymerized film momentarily turned light pink,but it immediately returned to the original deep reddish brown.

[0116] Next, an electric current of −1 macwas further galvanostaticallypassed until the amount of electricity passed reached 4 mC/cm², uponwhich the polymerized film momentarily turned light pink, but itimmediately returned to the original deep reddish brown.

[0117] Subsequently, an electric current of −1 mA was furthergalvanostatically passed until the amount of electricity passed reached4 mC/cm², upon which the polymerized film abruptly turned yellow, but itturned deep reddish brown after 1 hr.

[0118] While the embodiments of the present invention and the examplethereof have been described above, the embodiments and example can bemodified variously based on the technical thought of the invention.

[0119] For example, while description has been made paying attention topyrrole molecule and silver ion in the above embodiments and example,these are not limitative, and the kind of the acid to be used can easilybe changed; in addition, the substituent group for changing the basicitymay be selected as required.

[0120] Specifically, the 5-membered heterocyclic aromatic compound maycontain a B (boron) or P (phosphorus) atom other than the N (nitrogen)atom, but it is necessary to secure such a structure that these atomsform a soft base, and, for example, it is preferable that such an atomis protonated or takes an sp3 hybrid orbital.

[0121] In addition, it is preferable that the transition metal ion isacidic. Examples of the transition metal ion includes ions of atomsbelonging to Groups 1B to 5B of the periodic table. Specific examplesinclude not only Ag but also Cu and Au, which can be used preferably.Further, there may be used such atoms that the number of electrons inthe d-orbital becomes 10 upon ionization, and examples of such atomsinclude Zn, Cd, Hg, etc.

[0122] Besides, the metal complex of a heterocyclic aromatic compoundbased on the present invention preferably takes the form of a polymerwith other molecule; for example, a compound may be formed in which thepyrrole molecule itself or other molecule is substituted for or added toa part of the complex.

[0123] According to the metal complex of a heterocyclic aromaticcompound based on the present invention, the position of the centralatom (namely, the number of atoms (or the number of electrons) relatingto the coordination ability) can be regulated by the above-mentionedinternal or external factor, and the intrinsically unstable structurecan thereby be automatically stabilized. This means that a structuralchange of a derivative (inclusive of polymer) of the metal complex iseffected depending on the change in the position of the central atom,whereby the structure is stabilized, and, particularly, the metalcomplex or the derivative thereof can preferably function as a moleculardevice in technological fields.

1. A metal complex of a heterocyclic aromatic compound, comprising anacidic central atom comprised of a metal, and basic ligands comprised ofa 5-membered heterocyclic aromatic compound, wherein the position ofsaid central atom can be changed by an internal or external factor.
 2. Ametal complex of a heterocyclic aromatic compound as set forth in claim1, wherein the number of atoms relating to the coordination ability onthe ligand side can be regulated within range of 1 to 5 by the change inthe position of said central atom.
 3. A metal complex of a heterocyclicaromatic compound as set forth in claim 1, wherein said central atom iscomprised of an acidic transition metal ion.
 4. A metal complex of aheterocyclic aromatic compound as set forth in claim 3, wherein saidtransition metal ion is a silver ion.
 5. A metal complex of aheterocyclic aromatic compound as set forth in claim 1, wherein theposition of said central atom can be changed by transfer of an electriccharge as said internal factor or by application of an electric field ora change in acidity of the surrounding environment as said externalfactor.
 6. A metal complex of a heterocyclic aromatic compound as setforth in claim 1, which is a pyrrole-metal complex or a derivativethereof.
 7. A metal complex of a heterocyclic aromatic compound as setforth in claim 1, which is used for a functional material capable ofbeing used in a display, a molecular memory material, a refractive indexmodulation device, an analog switch, molecule recognition, or an ionpermeable film.